Installation and method for the treatment by membrane permeation of a gas stream with the methane concentration adjusted

ABSTRACT

Installation for membrane permeation treatment of feed gas flow containing at least methane and carbon dioxide including first, second, and third membrane separation units and at least one compressor B for aspirating the second permeate, a permeate from the first membrane separation unit being fed to the third membrane separation unit, a retentate from the first membrane separation unit being fed to the second membrane separation. After measuring the aspiration pressure and methane concentration of the second permeate before it is recycled to the feed, the pressure of the second permeate is adjusted according to the measured aspiration pressure and methane concentration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(a) and (b) to French patent application No. FR 1872938, filed Dec. 14,2018, the entire contents of which are incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to an installation for the treatment bymembrane permeation of a gas stream containing at least methane andcarbon dioxide in order to produce a methane-rich gas stream, of whichthe methane content meets the requirements of its use and to a methodfor controlling such an installation.

It relates in particular to the purification of biogas, with the aim ofproducing biomethane in accordance with the specifications for injectioninto a natural gas network.

Related Art

Biogas is the gas produced as organic matter breaks down in the absenceof oxygen (aerobic fermentation), also referred to as methanization.This may be natural breakdown—it is thus found in marshland or indischarges from household waste—but the production of biogas may alsoresult from the methanization of waste in a dedicated reactor referredto as a methanizer or digester.

Because of its chief constituents—methane and carbon dioxide—biogas is apowerful greenhouse gas; at the same time, it also constitutes a sourceof renewable energy that is appreciable in the context of the increasingscarcity of fossil fuels.

Biogas contains mainly methane (CH₄) and carbon dioxide (CO₂) inproportions that can vary according to the way in which it is obtained,but also contains, in smaller proportions, water, nitrogen, hydrogensulphide, oxygen and other organic compounds, in trace form.

Depending on the organic matter that has been broken down and on thetechniques used, the proportions of the components differ, although onaverage biogas contains, in the dry gas, from 30 to 75% methane, from 15to 60% CO₂, from 0 to 15% nitrogen, from 0 to 5% oxygen and tracecompounds.

Biogas is put to profitable use in various ways. It may, after lighttreatment, be put to profitable use near the production site in order tosupply heat, electricity or mixture of both (cogeneration); the highcarbon dioxide content reduces its calorific value, increases the costof compression and transport and limits the economic benefit of puttingit to profitable use in this way nearby.

Purifying the biogas to a greater degree allows it to be put to broaderuse, in particular, extensive purification of the biogas yields a biogasthat has been purified to the specifications of natural gas and whichcan be substituted for the latter; biogas thus purified is known as“biomethane”. Biomethane thus supplements the natural gas resources witha renewable proportion produced within the territories; it can be put toexactly the same uses as natural gas of fossil origin. It can be fedinto a natural gas network, a vehicle filling station; it can also beliquefied and stored in the form of liquefied natural gas (LNG) etc.

The ways in which the biomethane is put to profitable use are determinedaccording to the local context: local energy requirements, possibilitiesfor putting it profitable use as a biomethane fuel, and whether there isa natural gas transport or distribution network nearby, in particular.By creating synergy between the various parties operating in a giventerritory (agriculture, industry, civic authorities), the production ofbiomethane assists the territories in acquiring greater self-sufficiencyin terms of energy.

There are a number of stages that need to be gone through betweencollecting the biogas and obtaining the biomethane, the end-product thatcan be compressed or liquefied.

In particular, there are several stages needed prior to treatment whichis aimed at separating the carbon dioxide in order to produce a streamof purified methane. A first stage is to compress the biogas which hasbeen produced and brought in at atmospheric pressure, and thiscompression can be obtained—in the conventional way—using a compressor.The next stages are aimed at ridding the biogas of its corrosivecomponents which are hydrogen sulphide and the volatile organiccompounds (VOCs), the technologies used for this are; in theconventional way, pressure swing adsorption (PSA) and activated carboncapture. Next comes the stage which involves separating the carbondioxide in order ultimately to obtain methane at the purity required forits subsequent use.

Carbon dioxide is a contaminant typically present in natural gas and itis common practice to need to remove it. Varying technologies are usedfor this depending on the situation; among these, membrane technologyperforms particularly well when the CO₂ content is high; and it istherefore used for separating the CO₂ present in biogas originating fromreleased gases or plant or animal waste digesters.

Membrane gas-separation methods used for purifying a gas, whether theyemploy one or several membrane stages, need to be able to produce a gasat the required quality, at a low cost, while at the same timeminimizing the losses of the gas that is to be put to profitable use.Thus, in the case of biogas purification, the separation performed ischiefly a CH₄/CO₂ separation which needs to allow the production of agas containing, depending on its use, more than 85% CH₄, preferably morethan 95% CH₄, more preferably more than 97.5% CH₄, while minimizing theCH₄ losses in the residual gas and the cost of purification, the latterto a large extent being associated with the electricity consumption ofthe device that compresses the gas upstream of the membranes.

It is preferable for the natural gas network to receive a stream ofmethane at a methane concentration that is constant, so that theequipment that uses the biomethane can operate uniformly.

On that basis, one problem that arises is that of providing aninstallation that makes it possible to obtain a stream of methane at aconstant concentration.

SUMMARY OF THE INVENTION

One solution of the present invention is an installation for themembrane permeation treatment of a feed gas flow containing at leastmethane and carbon dioxide, comprising:

a compressor A for compressing the feed gas flow,

a first membrane separation unit able to receive the gas flow comingfrom the compressor and to supply a first permeate and a firstretentate,

a second membrane separation unit able to receive the first retentateand to supply a second permeate and a second retentate,

a third membrane separation unit able to receive the first permeate

and to supply a third permeate and a third retentate,

at least one measurement means for measuring the aspiration pressure ofthe second permeate in the second membrane unit, the at least onemeasurement means, typically being a pressure sensor,

at least one measurement means for measuring the methane CH₄concentration in the second retentate, the at least one measurementmeans for measuring the methane CH₄ concentration in the secondretentate being a CH₄ analyzer, and

at least one compressor B for aspirating the second permeate andadjusting the pressure of the second permeate according to theaspiration pressure measured and the methane concentration measuredbefore recycling the second permeate into the feed gas flow downstreamof the compressor A, with each membrane separation unit comprising atleast one membrane that is more permeable to carbon dioxide than tomethane.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an example of an installation according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Depending on the case, the installation according to the invention mayhave one or more of the following features:

-   -   the said installation comprises at least one pressure adjustment        means for adjusting the pressure of the feed gas flow depending        on the CH₄ concentration measured;    -   the adjustment means for adjusting the pressure of the feed gas        flow is a compressor or a progressive shut-off and pressurizing        valve,    -   the third retentate is recycled to the compressor used for        compressing the feed gas flow;    -   the membranes used in the membrane separation units have the        same selectivity;    -   at least one membrane separation unit comprises at least two        membranes with different selectivities.

The present invention also relates to a method for controlling aninstallation as defined in the invention, comprising the followingsteps:

-   -   a. a step of measuring the pressure of the second permeate,    -   b. a step of measuring the methane CH₄ concentration in the        second retentate,    -   c. a step of comparing the pressure measured in step a) and the        concentration measured in step b) against setpoint values, and        of determining the discrepancy with respect to these setpoint        values, and    -   d. a step of adjusting the pressure of the second permeate using        the compressor B in order to keep the CH₄ concentration value in        the second retentate constant.

As the case may be, the method according to the invention can exhibitone or more of the following features:

-   -   in step d), both the pressure of the second permeate is adjusted        using the compressor B and the pressure of the feed gas flow is        adjusted using the compressor A or using a progressive cut-off        and pressurizing valve;    -   the adjustment of the pressure of the feed gas flow involves        increasing or decreasing the pressure;    -   in the adjustment step, the compressor B is accelerated or        decelerated; note that an acceleration of the compressor B will        lead to a decrease in the pressure level in the membranes, and a        deceleration of the compressor B will lead to an increase in the        pressure level in the membranes,    -   the comparison step and the adjustment step are performed        automatically by data transmission and data processing means    -   the feed gas flow is biogas.

A data transmission and data processing means may for example be anindustrial processor of the programmable controller type.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives;modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing i.e.anything else may be additionally included and remain within the scopeof “comprising.” “Comprising” is defined herein as necessarilyencompassing the more limited transitional terms “consisting essentiallyof” and “consisting of”; “comprising” may therefore be replaced by“consisting essentially of” or “consisting of” and remain within theexpressly defined scope of “comprising”.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

The invention claimed is:
 1. A method for controlling an installationfor the membrane permeation treatment of a feed gas flow containing atleast methane and carbon dioxide, comprising the steps of: providing aninstallation comprising: a compressor A for compressing a combination ofthe feed gas flow and a third retentate, a first membrane separationunit able to receive the gas flow coming from the compressor and tosupply a first permeate and a first retentate, a second membraneseparation unit able to receive the first retentate and to supply asecond permeate and a second retentate, a third membrane separation unitable to receive the first permeate and to supply a third permeate andthe third retentate, at least CH₄ analyzer for measuring the methane CH₄concentration in the second retentate, and at least one compressor B foraspirating the second permeate, adjusting the pressure of the secondpermeate according to the aspiration pressure measured and the methaneconcentration measured before recycling the second permeate into thefeed gas flow downstream of the compressor A, and combining thepressure-adjusted second permeate with the compressed combined flows ofthe feed gas and the third retenate, wherein each membrane separationunit comprising at least one membrane that is more permeable to carbondioxide than to methane, measuring the pressure of the second permeateat a location between the second membrane separation unit and thecompressor B; measuring the methane CH₄ concentration in the secondretentate; comparing the measured pressure and the measured CH₄concentration against associated pressure and CH₄ concentration setpointvalues and determining a difference between the measured pressure andand CH₄ concentration and the associated setpoint values; and adjustingthe pressure of the second permeate using the compressor B in order toobtain a concentration of CH₄ in the second retentate corresponding tothe CH₄ concentration setpoint.
 2. The method of claim 1, wherein, instep d), both the pressure of the second permeate is adjusted using thecompressor B and the pressure of the feed gas flow is adjusted using thecompressor A or using a progressive cut-off and pressurizing valve. 3.The method of claim 2, wherein the adjustment of the pressure of thefeed gas flow involves increasing or decreasing the pressure.
 4. Themethod of claim 1, wherein, in the adjustment step, the compressor B isaccelerated or decelerated.
 5. The method of claim 4, wherein thecomparison step and the adjustment step are performed automatically bydata transmission and data processing means.
 6. The method of claim 1,wherein the feed gas flow is biogas.